It is important to realise that a lot of what we know about the structure of atoms has been developed over a long
period of time. This is often how scientific knowledge develops, with one person building on the ideas of someone
else. We are going to look at how our modern understanding of the atom has evolved over time.
The idea of atoms was invented by two Greek philosophers, Democritus and Leucippus in the fifth century BC. The
Greek word ατoμoν (atom) means indivisible because they believed that atoms
could not be broken into smaller pieces.
Nowadays, we know that atoms are made up of a positively charged nucleus in the centre
surrounded by negatively charged electrons. However, in the past, before the structure of the
atom was properly understood, scientists came up with lots of different models or
pictures to describe what atoms look like.
A model is a representation of a system in the real world. Models help us to understand systems and their
For example, an atomic model represents what the structure of an atom could look like, based on
what we know about how atoms behave. It is not necessarily a true picture of the exact structure of an atom.
Models are often simplified. The small toy cars that you may have played with as a child are models. They give
you a good idea of what a real car looks like, but they are much smaller and much simpler. A model cannot always
be absolutely accurate and it is important that we realise this, so that we do not build up an incorrect idea
Dalton's model of the atom (ESAAO)
John Dalton proposed that all matter is composed of very small things which he called atoms. This was not a
completely new concept as the ancient Greeks (notably Democritus) had proposed that all matter is composed of
small, indivisible (cannot be divided) objects. When Dalton proposed his model electrons and the nucleus were
Thomson's model of the atom (ESAAP)
After the electron was discovered by J.J. Thomson in 1897, people realised that atoms were made up of even
smaller particles than they had previously thought. However, the atomic nucleus had not been discovered yet and
so the “plum pudding model” was put forward in 1904. In this model, the atom is made up of negative
electrons that float in a “soup” of positive charge, much like plums in a pudding or raisins in a
fruit cake (Figure 4.3). In 1906, Thomson was awarded
the Nobel Prize for his work in this field. However, even with the Plum Pudding Model, there was still no
understanding of how these electrons in the atom were arranged.
The discovery of radiation was the next step along the path to building an accurate picture of
atomic structure. In the early twentieth century, Marie and Pierre Curie, discovered that some elements (the
radioactive elements) emit particles, which are able to pass through matter in a similar way to
X–rays (read more about this in Grade 11). It was Ernest Rutherford who, in 1911, used this discovery to
revise the model of the atom.
Two other models proposed for the atom were the cubic model and the Saturnian model. In the cubic model, the
electrons were imagined to lie at the corners of a cube. In the Saturnian model, the electrons were imagined
to orbit a very big, heavy nucleus.
Rutherford's model of the atom (ESAAQ)
Rutherford carried out some experiments which led to a change in ideas around the atom. His new model described
the atom as a tiny, dense, positively charged core called a nucleus surrounded by lighter, negatively charged
electrons. Another way of thinking about this model was that the atom was seen to be like a mini solar system
where the electrons orbit the nucleus like planets orbiting around the sun. A simplified picture of this is
shown alongside. This model is sometimes known as the planetary model of the atom.
Bohr's model of the atom (ESAAR)
There were, however, some problems with Rutherford's model: for example it could not explain the very
interesting observation that atoms only emit light at certain wavelengths or frequencies. Niels Bohr solved this
problem by proposing that the electrons could only orbit the nucleus in certain special orbits at different
energy levels around the nucleus.
James Chadwick (ESAAS)
Rutherford predicted (in 1920) that another kind of particle must be present in the nucleus along with the
proton. He predicted this because if there were only positively charged protons in the nucleus, then it should
break into bits because of the repulsive forces between the like-charged protons! To make sure that the atom
stays electrically neutral, this particle would have to be neutral itself. In 1932 James Chadwick discovered the
neutron and measured its mass.
Other models of the atom (ESAAT)
Although the most commonly used model of the atom is the Bohr model, scientists are still developing new and
improved theories on what the atom looks like. One of the most important contributions to atomic theory (the
field of science that looks at atoms) was the development of quantum theory. Schrodinger, Heisenberg, Born and
many others have had a role in developing quantum theory.
Models of the atom
Textbook Exercise 4.1
Match the information in column A, with the key discoverer in column B.
1. Discovery of electrons and the plum pudding model